1. Field of the Invention
The invention relates to machines having a wire part for the manufacture of fibrous material webs, particularly for a papermaking machine wherein a web is guided between first and second continuous wire belts in a region of a twin-wire zone having a supporting surface and wherein at least a part of the supporting surface forms a supporting region which supports the first wire belt at an inner surface thereof and the second wire belt at an outer surface thereof.
2. Description of Related Technology
Wire parts of machines for the production of fibrous material webs, especially for papermaking machines with an integrated double-wire zone, have been known in various embodiments which are based on stringing together different variations of dewatering elements. For example, WO 91/02842 discloses a double-wire zone in which the dewatering of the fibrous material suspension is done between two continuous wire belts on dewatering elements combined with one another in different ways. Such dewatering elements are forming cylinders, forming shoes and embodiments with non-rigidly supported and rigid strips which are disposed alternately on the two sides of the two wire belts. The two wire belts with the fibrous material suspension between them are guided along these dewatering elements in order to dewater the suspension and form a web, whereby, due to the geometrical design of the individual dewatering elements, the consequently occurring wire tensions lead to intensification of the dewatering. The arrangement of non-rigidly supported strips, preferably on the lower wire, which are displaced to the gaps of the rigidly arranged strips that engage with the upper wire, results in the introduction of forces on the inner surface of the lower wire, which are manifested in line-like loads, which, due to the minimal cross-sectional changes and due to the high working speed, produce turbulences in the fibrous material suspension and prevent the formation of flocks. A disadvantage of the known arrangement is that, due to the relative movement between the upper wire and the rigid strips, the inner surface of the upper wire is subjected to wear phenomena. Also, the wire part has to be constructed quite long as a result of the sequential arrangement of the various dewatering elements.
An embodiment of a paper machine wire part is known from unpublished German Patent Application No. P 43 01 103 which avoids the disadvantages of the wire part disclosed in WO 91/02842. Supporting bodies are provided as dewatering elements, arranged in the double-wire zone, with pressure elements that act non-rigidly against their [of the wires'] supporting surface. Here, mainly supporting elements with continuous supporting surface come into consideration as supporting bodies, for example, forming cylinders and belts. This offers the advantage that the effects of centrifugal forces and the resulting pressure pulses can be added and thus dewatering is significantly promoted. Friction on the wire belt is largely avoided which is manifested in a reduction of wear and also in a reduction of the required driving forces. Supporting surfaces are chosen which are rigid in the direction of action of the pressure and which cannot deviate on the wire belts in the direction of pressure. The pressing of the pressure elements provides the advantage that line-like or point-like loads, and in the case of a corresponding design of the pressure element, even area-like loads are applied onto the wire belt which again results in desirable microturbulence in the fiber suspension between the wire belts. The pressure guided by the pressure element onto the inner surface of the wire belt can be varied, that is, can be adjusted so that it can be variably altered. The variation of the pressure applied by the pressure elements onto the inner surface of the wire can be continuous or step-wise. In order to adapt to changing operating conditions, the adjustment of the pressure value applied by the pressure elements can be integrated into a control or control system.
The disadvantages of the embodiments named in WO 91/02842 and P 43 01 103 consist mainly in the fact that the dewatering and the direction thereof are very difficult to control. Due to the elimination of water caused by the wire tension, damage of the fibrous material suspension occurs on the curved supporting bodies, and these are reflected by schlieren (streaks) in the finished fibrous material web. Another disadvantage is that no active influence can be exerted on the symmetrical formation of the fibrous material web or on control of the filler distribution.